I have had to fit safety switches and light curtains etc for some companies, usually due to a strong union presence that insist on management upgrading machine safety.
With a CNC machine you have to stop both axis and spindle movement, apart from the safety aspect, if you just stopped the spindle it would result in a tool crash for sure.
I usually have fitted safety switches these are special switches for this application, they usually come red in colour and have a switch operator that requires the sequence be correct, this avoids anyone jimmying an over ride.
The machine program stop can be interlocked with the switches and also prevents a restart.
Also most CNC's allow operator messages, and I include one just to advise the operator the reason for the stoppage.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.